Advanced Terahertz Scanning Near-Field Optical Microscope Enhances Quantum Computing Circuitry

Image credit: Visualization depicts the microscope’s tip illuminating the material with terahertz radiation. The variations in color on the material correspond to light-scattering data, while the red and blue lines indicate the terahertz waves. Courtesy: U.S. Department of Energy Ames National Laboratory

Researchers employed an advanced terahertz scanning near-field optical microscope (SNOM) for identifying imperfections in circuits dedicated to quantum computing, particularly focusing on nano-sized Josephson Junctions. The rectification of such faults is crucial for capitalizing on the rapid data processing capabilities intrinsic to quantum computing.

A collaboration between scientists from the U.S. Department of Energy’s (DOE) Ames National Laboratory and the Superconducting Quantum Materials and Systems Center (SQMS), a National Quantum Information Science Research Center spearheaded by Fermilab, employed the terahertz SNOM microscope—originally conceived at Ames Lab—to scrutinize the nanoscale Josephson Junction (JJ) connectivity and interfaces.

Produced by Rigetti Computing, an SQMS associate, the JJ is a pivotal element in the functionality of superconducting quantum computers. The JJ serves as the origin for a two-level system at extreme low temperatures, thus creating a quantum bit or qubit. Images captured using the terahertz microscope disclosed a flawed boundary within the nano junction, resulting in disrupted conductivity and thereby posing a barrier to maintaining extended coherence durations crucial for quantum calculations.

Decoding the Nature of Qubits

Quantum computers employ quantum bits, colloquially known as qubits, which operate in a manner analogous to digital computer bits. Bits in a traditional computer serve as the smallest data units for processing and storage, existing in binary states—either 0 or 1. Qubits, on the other hand, can coexist as both 0 and 1 due to their quantum nature, thereby granting quantum computers superior data processing speed compared to their classical counterparts.

Images captured via the terahertz SNOM illustrated issues with electrical field concentration and asymmetry, confirming connectivity complications within the JJ. Courtesy: U.S. Department of Energy Ames National Laboratory

The key to improved qubits lies in comprehending the functionality of the nano Josephson Junction. Jigang Wang, an Ames Lab scientist and the project’s lead researcher, elaborated that this JJ ensures the stable, non-dissipative flow of supercurrent at cryogenic temperatures, thereby permitting qubits to maintain their quantum states.

Addressing Technical Obstacles and Innovations

Wang noted that the intricate structure in quantum circuits often leads to localized electric field concentration, thereby causing scattering, energy loss, and ultimately, decoherence. The present challenge for the industry lies in minimizing this decoherence.

Utilizing the terahertz SNOM microscope, which was previously developed at Ames Lab, Wang’s team recorded images of the JJ under conditions of electromagnetic field coupling. The high-resolution capabilities of the microscope were achieved without compromising the structural integrity of the junction component. These images revealed a disconnection within the JJ, signifying an area for potential improvement in circuit fabrication.

Two significant conclusions were derived from these findings: first, the identification of a fault within the JJ fabrication process, which Rigetti Computing can now address to enhance their quantum circuit quality, and second, establishing the terahertz SNOM microscope as a valuable instrument for scrutinizing quantum circuit elements with high throughput.

Prospective Aims and Microscope Capabilities

Quantum circuits typically operate at ultra-low cryogenic temperatures. Wang’s team had previously shown that the terahertz SNOM could function under these extreme conditions. “The ultimate objective is to continue refining this sophisticated cryogenic terahertz SNOM instrument to monitor supercurrent tunneling in real-time and real-space conditions of an operational qubit,” Wang stated.

Furthermore, Wang stressed the critical role of Ames Lab’s involvement in the SQMS community. He conveyed gratitude for the collaborative efforts, acknowledging that solving such complex technological and scientific problems truly requires a multi-disciplinary team. “Being a part of Ames Lab has significantly contributed to advancing the national quantum initiative,” Wang noted.

Reference: “Visualizing heterogeneous dipole fields by terahertz light coupling in individual nano-junctions” authored by Richard H. J. Kim, Joong M. Park, Samuel Haeuser, Chuankun Huang, Di Cheng, Thomas Koschny, Jinsu Oh, Cameron Kopas, Hilal Cansizoglu, Kameshwar Yadavalli, Josh Mutus, Lin Zhou, Liang Luo, Matthew J. Kramer & Jigang Wang, published on 22 June 2023 in Communications Physics.
DOI: 10.1038/s42005-023-01259-0

Frequently Asked Questions (FAQs) about Terahertz SNOM Microscope in Quantum Computing

More about Terahertz SNOM Microscope in Quantum Computing

• U.S. Department of Energy’s Ames National Laboratory
• Superconducting Quantum Materials and Systems Center (SQMS)
• Fermilab’s Quantum Information Science
• Rigetti Computing
• Quantum Computing Fundamentals
• Josephson Junction in Quantum Computing
• Original Article: Visualizing heterogeneous dipole fields by terahertz light coupling in individual nano-junctions